Phase shift the colour burst!

after thinking about @schaferob’s chroma-luma inverter circuit where the colours can be inverted by inverting the colour-burst i started wondering how one would go about phase shifting this burst to achieve variable chroma offset ? - this effect is used often in feedback loops with software or prebuilt video-processors.

i gather there are transistor circuits that can do this , and the effect is popular with guitar players

any good examples out there for video circuits ? @BastienL

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Looking at audio phasers, there is 3 main variations:

  • Transistor based like the Univibe, vactrols (LED+LDR) are used to control the corner frequency with a voltage.

  • Op-amp based like the MXR Phase 90, vactrols or alternatively JFET as voltage controlled resistors are used to have control over the frequency. The filter topology is called “all-pass”, cause it does pass all the frequency at equal gain (as opposed to other filters) while still shifting the phase.

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  • OTA based phasers, as the Small Stone phaser, same principle as the previous one, but the op-amps are replaced by transconductance amplifiers, which gain can be controlled with current, thus eliminating the need for vactrols or JFETs.

About video now, I’ve came accross a transistor based one:

(from ProgettoElektor_1987_07-08 Color Processor.pdf (1.7 MB) )

Then there is this really fancy one from one of LT application notes:

(LT Application Note 57.pdf (545.8 KB) )

Then I couldn’t find much more iterations of phase shifters for video, I remember Lars from LZX evocating a solution with varactors, those are basically diodes which capacity changes depending of the voltage going across it.

Also, most of these designs goal is to have the phaseshift voltage controlled, to be used with an internal LFO (in case of guitar pedals) or external voltage, so it makes things a bit easier if you don’t need the voltage control and only have the hue set with a potentiometer.

However, I think @schaferob will be better at answering this though, as I’ve seen his recent post about a phase shifter he is working on, based on LT1228 (op amp with gain control, which can be considered as a “video rate OTA”).

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Cool idea. There’s also one more control pin on the 74HC4051 that (if carefully used) would allow for some modulation/variation. There’s the possibility of reducing the level of the burst to make the colors more saturated.

I was playing with phase shifting using an LT1228 but I only got about a 60 degree range with it. (And it’s pricey.)

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